Method and apparatus for compensating for a parameter change in a synthetic aperture imaging system

ABSTRACT

There is described a method for processing data generated by a synthetic aperture imaging system, comprising: receiving raw data representative of electromagnetic signals reflected by a target area to be imaged; receiving a parameter change for the synthetic aperture imaging system; digitally correcting the raw data in accordance with the parameter change, thereby compensating for the parameter change in order to obtain corrected data; and generating an image of the target area using the corrected data.

TECHNICAL FIELD

The present invention relates to the field of synthetic aperture imagingsystems, and more particularly to methods and apparatuses forcompensating for a change of parameter in a synthetic aperture imagingsystem.

BACKGROUND

Synthetic aperture radar (SAR) imaging systems are widely used in aerialand space reconnaissance. Usually, an aircraft or a spacecraft isprovided with a SAR imaging system which transmits radar pulses andcollects radar echoes corresponding to the radar pulses reflected by anobject to be imaged.

Due to the large amount of data generated by a SAR system, opticalsolutions have been developed for processing the SAR raw data. Forexample, the SAR raw data can be recorded on a photosensitive film, oran optical image of the SAR raw data can be generated using a lightmodulator. However, in such optical processing systems, the position ofoptical components has to be changed in order to compensate forparameters changes for the SAR imaging system, such as an altitudechange for example. The requirement for moving the optical componentsreduces the sturdiness and viability for the optical SAR raw dataprocessing system.

Therefore there is a need for an improved method and apparatus forcompensating for a parameter change in a SAR imaging system.

SUMMARY

In accordance with a first broad aspect, there is provided a method forprocessing data generated by a synthetic aperture imaging system,comprising: receiving raw data representative of electromagnetic signalsreflected by a target area to be imaged; receiving a parameter changefor the synthetic aperture imaging system; digitally correcting the rawdata in accordance with the parameter change, thereby compensating forthe parameter change in order to obtain corrected data; and generatingan image of the target area using the corrected data.

In accordance with a second broad aspect, there is provided a system forgenerating a synthetic aperture image of a target area, comprising: amemory for storing raw data representative of electromagnetic signalsreflected by said target area and a parameter change for said syntheticaperture imaging system; a raw data correcting module adapted to correctsaid raw data in accordance with said parameter change in order tocompensate for said parameter change and obtain corrected data; and animage generator adapted to generate an image of said target area usingsaid corrected data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a SAR imaging system for imaging a target area, inaccordance with an embodiment;

FIG. 2 is a flow chart illustrating a method for generating a SAR imagein accordance with a SAR parameter change, in accordance with anembodiment;

FIGS. 3A and 3B illustrate graphical representations of SAR raw data andSAR corrected data, respectively, in accordance with an embodiment; and

FIG. 4 is a block diagram illustrating a system for generating a SARimage in accordance with a SAR parameter change, in accordance with anembodiment.

DESCRIPTION

FIG. 1 illustrates one embodiment of a satellite 10 provided with a SARimaging system. The satellite 10 is in orbit around an object to beimaged, such as a planet for example. The satellite 10 is travelingalong a satellite flight path while imaging the planet. The SAR imagingsystem is adapted to emit successive electromagnetic radar pulses 12 indirection of the planet. Each radar pulse 12 is characterized by a pulseduration T and two successive radar pulses 12 are temporally spacedapart by an inter-pulse duration ΔT 14. The pulse duration T and theinter-pulse duration ΔT 14 defines a pulse repetition frequency whichcorresponds to the repetition rate of the outgoing radar pulses 12. Theemitted radar pulses 12 form a radar beam which illuminates the targetarea to be imaged. The area of the planet ground which intersects theradar beam is referred to as the footprint 16 of the radar beam. WhileFIG. 1 illustrates an oval footprint 16, it should be understood thatthe footprint 16 may have other shape. For example, the footprint 16 maybe round. While the satellite 10 is moving along the satellite flightpath, the footprint 16 is moving, thereby defining a swath 18. The swath18 is characterized by a length in an azimuth direction and a width in arange direction. The azimuth direction corresponds to the propagationdirection of the radar beam, i.e. the flight path direction, and therange direction is the direction normal to the azimuth direction. Whenreaching the ground, the radar pulses 12 are reflected to give rise toradar echoes. The radar echoes are collected by the SAR system andprocessed in order to generate a radar image of the target area.

The SAR system mounted to the satellite 10 is provided with at least oneemitting antenna for emitting the pulses 12. The emitting antenna can beused for detecting the radar echoes reflected by the ground.Alternatively, at least one receiving antenna different from theemitting antenna can be used for collecting the radar echoes.

A SAR image is generated by superposing a plurality of radar echoeswithin the range and azimuth of the SAR antenna footprint 16. Thereceived echoes are converted into electrical signals which are referredto as SAR raw data. Alternatively, the electrical signals may besubsequently converted into digital data, which are also referred to asthe SAR raw data. The SAR raw data is then processed to create the SARimage of the illuminated target area. A high resolution in the azimuthdirection is achieved by applying SAR signal processing withoutrequiring large antennas. The SAR signal processing allows synthesizinga large aperture antenna. SAR signal processing can be mathematicallydescribed as a correlation or a filtering process on all of the radarechoes received during an aperture time.

During operation of a SAR imaging system, at least one of the parametersof the SAR imaging system may vary. In order to generate an adequateimage, a compensation for the parameter variation must be performed.Examples of SAR imaging system parameters which may vary compriseparameters of which a variation causes a Doppler shift, parameters ofwhich a variation causes a variation of the Fresnel zone plate focallength, and the like. The Doppler shift refers to a shift of the Dopplercentroid which is the center Doppler frequency or null of the Dopplerspectrum as the radar beam sweeps past the target area. Because agraphical representation of SAR raw data can be thought of as an opticalinterference pattern, an interference pattern parameter can be defined.In the case of a stripmap SAR, a Fresnel zone plate may be defined andassociated with SAR raw data, and the interference pattern parameter isa Fresnel zone plate focal length. For a spotlight SAR, a Fourier slicemay be defined and associated with SAR raw data, and the interferencepattern is a Fourier slice scale parameter. An example of a parameter ofwhich a variation causes a Doppler shift is the squint angle whichcorresponds to the angle between the radar beam center and the normal tothe flight path. Examples of a parameter of which a variation causes avariation of the interference pattern parameter, such as the Fresnelzone plate focal length or the Fourier slice scale parameter, are thepulse repetition frequency, the range sampling frequency, the slantrange distance, and the like. The range sampling frequency or radarsampling frequency is defined as the number of pulses transmitted persecond by the SAR system. The pulse repetition frequency is thefrequency at which the SAR system samples the radar return signal (echo)from the ground. The slant range distance corresponds to the distancebetween the SAR radar system and the target area to be imaged.

FIG. 2 illustrates one embodiment of a method 20 for processing SAR rawdata generated by a SAR imaging system in which compensation for a SARimaging system parameter is digitally performed. The first step 22comprises receiving SAR raw data. The SAR raw data is a digitalrepresentation of the echoes reflected by the target area to be imagedand received by the SAR system during a determined period of time. TheSAR raw data comprises amplitude and phase information. The second stepcomprises receiving a parameter change for the SAR radar system that hasbeen performed before the radar illumination of the target area to beimaged. The third step 26 of the method 20 comprises digitallycorrecting the SAR raw data in accordance with the SAR parameter change.By digitally correcting the SAR raw data, a compensation for theparameter change is performed onto the SAR raw data and corrected datais obtained. The last step 28 of the method 20 comprises generating aSAR image using the corrected data.

In one embodiment, the SAR parameter change received at step 24 causes avariation of the interference pattern parameter, such as the Fresnelzone plate focal length for a stripmap SAR or the Fourier slice scaleparameter for a spotlight SAR, for example. Such a parameter change canbe a change of the pulse repetition frequency, the range samplingfrequency, the slant range distance, or the like. In this case, step 26of digitally correcting the SAR raw data comprises determining a scalingfactor in accordance with the interference pattern parameter change andapplying the scaling factor to the SAR raw data, thereby obtaining thecorrected data. Applying a scaling vector to the SAR raw data can bethought of as applying a translation vector to the SAR raw data points.It should be understood that the value of the scaling factor and thetranslation vector may vary depending on the range and/or azimuth of theSAR raw data points.

The application of a scaling can be mathematically represented by theEquation 1:ss(x,y)=A·dd(x,y)  Eq. 1where ss(x, y), A, and dd(x, y) respectively represent the SAR scaleddata, the scaling vector, and the SAR raw data.

In one embodiment, a scaling factor corresponding to a translation alongthe azimuth axis is applied to the SAR raw data. In this case, the valueof the scaling factor may vary as a function of the position along therange axis for the SAR data points. In another embodiment, a scalingfactor corresponding to a translation along the range axis is applied tothe SAR raw data. In this case, the value of the scaling factor may varyas a function of the position along the azimuth axis for the SAR datapoints. In a further embodiment, a scaling vector corresponding to atranslation along both the azimuth and range axes is applied to the SARraw data. If the scaling factor has the same value for both the azimuthand range axes such that the same translation vector is applied alongboth the azimuth and range axes, applying a scaling vector can bethought of as a zoom-in or a zoom-out. Alternatively, the value of thescaling vector may be different for the azimuth and the range axes.

FIG. 3A illustrates one embodiment of a graphical representation ofstripmap SAR raw data illustrative of electromagnetic signals reflectedby a target area to be imaged. The SAR raw data are graphicallyrepresented by a central disc and circles which are concentric with thecentral disc. The graphical representation of the SAR raw data can bethought of as an interference pattern in optics. Because a SAR parameterchange causing a variation of the Fresnel zone plate focal lengthoccurred before acquiring the SAR raw data illustrated in FIG. 3A, acorrection of the SAR raw data is performed for compensating for the SARparameter change. A scaling factor is determined in accordance with theSAR parameter change. In the present case, applying the scaling factorto the SAR raw data corresponds to applying a translation vectordirected towards the center of the central disc to the central disc andthe concentric circles. The value of the translation vector varies alongthe azimuth axis such that the central disc and the concentric circlesare converted into ovals as illustrated in FIG. 3B.

In one embodiment, the variation of the interference pattern parameter,such as the Fresnel zone plate focal length for example, is firstdetermined, and the value of the scaling factor is determined using thevariation of the Fresnel zone plate focal length. In this case, thescaling factor in the azimuth and range axis is equal to the square rootof the variation of the azimuth Fresnel zone plate focal length and thesquare root of the variation of the range Fresnel zone plate focallength, respectively.

In one embodiment, the scaling factor is determined in accordance with adatabase of empirical data previously stored in a memory. The empiricaldata contains scaling factor values as a function of the SAR parameterchange value. Alternatively, the database of empirical data comprisesvalues of interference pattern parameter variations, such as Fresnelzone plate focal length variations or variations for the Fourier slicescaling parameter for example, as a function of the SAR parameter changevalue and the scaling factor value is determined using the value for theFresnel zone plate focal length variation.

In another embodiment, the scaling factor is determined from actual andnominal values for SAR parameters of the SAR system. A nominal SARparameter value refers to the value that the corresponding SAR parametershould have according to the design of the SAR system. The scalingfactor is calculated as the square root of the ratio of the currentFresnel zone plate focal length to the nominal Fresnel zone plate focallength. The scaling factor for the range and the azimuth direction isgiven by Equations 2 and 3, respectively:

$\begin{matrix}{{ScalingFactor}_{Range} = \sqrt{\frac{ChirpRate}{ChirpRateNomial} \cdot \frac{RangeSamplingFrequencyNominal}{RangeSamplingFrequency}}} & {{Eq}.\mspace{14mu} 2} \\{{ScalingFactor}_{Azimuth} = \sqrt{\frac{SlantRangeNominal}{{SlantRange}\mspace{11mu}({RangeSwathNumber})} \cdot \frac{PRFNominal}{PRF}}} & {{Eq}.\mspace{14mu} 3}\end{matrix}$where the Chirp Rate corresponds to the instantaneous rate of change ofthe frequency of the emitted signal, the Range Swath Number is thenumber of the range swath, and PRF is the pulse repetition frequency.

In another embodiment, the SAR parameter change received at step 24causes a Doppler shift. Such a parameter change can be a change of thesquint angle, a change of the eccentricity of the platform orbit, or thelike. In one embodiment, SAR raw data can be represented by a complexfunction. In this case, step 26 of correcting the SAR raw data comprisesdetermining a phase factor in accordance with the SAR parameter changeand applying the phase factor to the complex function, thereby obtainingthe corrected data. Alternatively, the Doppler shift induced by the SARparameter change is determined and the phase factor is then determinedusing the Doppler shift. Any adequate method for determining the Dopplershift can be used. For example, if the SAR imaging system is providedwith an adequate number of echo detectors, a Fourier transform of theSAR raw data may be performed. The Doppler shift can be determined fromthe Fourier transform by determining a shift of the center of theFourier transform.

Applying the phase factor to the SAR raw data consists in multiplyingthe SAR raw data function ff(x, y) by an imaginary number exp[jφ], φbeing the previously determined phase factor.

$\quad\begin{matrix}\begin{matrix}{{{gg}\left( {x,y} \right)} = {{{ff}\left( {x,y} \right)}{\exp\left\lbrack {j\;\varphi} \right\rbrack}}} \\{= {{{{ff}\left( {x,y} \right)}}{\exp\left\lbrack {j\;{{\Phi\Phi}\left( {x,y} \right)}} \right\rbrack}{\exp\left\lbrack {j\;\varphi} \right\rbrack}}} \\{= {{{{ff}\left( {x,y} \right)}}{\exp\left\lbrack {j\left( {{{\Phi\Phi}\left( {x,y} \right)} + \varphi} \right)} \right\rbrack}}}\end{matrix} & {{Eq}.\mspace{14mu} 4}\end{matrix}$where gg(x, y), |ff(x, y)|, and jΦΦ(x, y) respectively represent the SARcorrected data function, the amplitude function of the SAR raw data, andthe phase function of the SAR raw data.

In another embodiment, the SAR raw data is graphically represented by aSAR pattern such as the SAR pattern illustrated in FIG. 3A. SAR raw datapoints forming the SAR pattern are organized in rows along the azimuthaxis and in columns along the range axis. Each SAR raw data point isassociated with an amplitude value and a phase value. In this case, step26 of correcting the SAR raw data comprises determining an azimuth shiftand shifting the columns of SAR raw data points in accordance with thedetermined azimuth shift. Alternatively, the Doppler shift induced bythe SAR parameter change can be determined using any adequate method andthe azimuth shift is determined using the Doppler shift. It should beunderstood that applying a linear phase vector to the complex functionrepresenting the SAR raw data is equivalent to applying an azimuth shiftto a SAR pattern representing the SAR raw data.

In one embodiment, the scaling factor and/or the column shift and/or thephase factor are determined using a database of empirical data. Thedatabase contains values for the scaling factor and/or the column shiftand/or the phase factor as a function of SAR parameter change values,interference pattern parameter change values, such as azimuth and/orrange Fresnel zone plate focal length values or Fourier slice scalingparameter values for example, and/or Doppler shift values.

In one embodiment, the Doppler shift is determined using an estimatedDoppler centroid value. An estimation for the Doppler centroid is givenby Equation 5:

$\begin{matrix}{f_{DC} = {\frac{U}{\lambda_{radar}} - \frac{2\;{V_{g} \cdot \tan}\;\theta_{sq}}{\lambda_{radar}}}} & {{Eq}.\mspace{14mu} 5}\end{matrix}$

where U is the planet's rotational velocity, V_(g) is the satellite'sprojected ground velocity, λ_(radar) is the radar wavelength, and θ_(sg)is the squint angle.

In one embodiment, step 28 of generating the SAR image using thecorrected data comprises digitally generating the SAR image using thecorrected digital data. Any adequate digital method for generating a SARimage using the corrected data can be used. Examples of adequate digitalmethods comprise the range/Doppler method, the wavenumber method, thechirp-scaling method, the plane-wave approximation method, and the like.

In another embodiment, step 28 of generating the SAR image using thecorrected data comprises optically generating the SAR image using thecorrected digital data. Any adequate optical methods for generating aSAR image using the corrected SAR data can be used. The SAR image isgenerated by creating an optical image of the SAR raw data and opticallyprocessing the image of the SAR raw data.

In one embodiment, the optical image of the SAR raw data is createdusing a light modulator such as a spatial light modulator (SLM) or amicro display. The light modulator comprises addressable pixelsorganized in rows along a first axis representing the azimuth and incolumns along a second axis representing the range. The transmittance ofeach pixel of the light modulator is controlled in accordance with theamplitude and/or phase of the corresponding SAR corrected data value. Acoherent light such as a laser light is generated and illuminates thelight modulator. The incident laser light is modulated by the lightmodulator and an optical image of the SAR corrected data is generated atthe output of the light modulator. The optical image of the SARcorrected data is then optically processed to reconstruct the SAR imageof the target area. The reconstructed SAR image can be projected on ascreen. Alternatively, an optical sensor such as a charge-coupled device(CCD) or a complementary metal-oxide-semiconductor (CMOS) sensor, forexample, can be used to convert the optical SAR image of the target areainto a digital SAR image of the target area which can be recorded in amemory or sent to a base station, for example.

In one embodiment, the SLM comprises two SLMs mapped one against theother. The transmittance of the pixels of the first SLM is set inaccordance with the amplitude of the SAR raw data points and thetransmittance of the pixels of the second SLM is set in accordance withthe phase of the SAR raw data points, or vice versa.

FIG. 4 illustrates one embodiment of a SAR processing system 40 whichcomprises a memory 42, a SAR raw data correcting module 44, and a SARimage generator 46. The SAR processing system 40 is adapted to receiveSAR raw data from the SAR imaging system and a parameter change for theSAR imaging system. The SAR raw data and the parameter change are storedin the memory 42. The SAR raw data correcting module 44 accesses the SARraw data and the parameter change from the memory. The SAR raw datacorrecting module 44 is adapted to generate corrected SAR data inaccordance with the method described above. The SAR corrected data arethen transmitted to the SAR image generator 46 which is adapted togenerate a SAR image using the SAR corrected data. Alternatively, theSAR corrected data is stored in the memory 42 and the SAR imagegenerator 46 accesses the SAR corrected data from the memory 42.

In one embodiment, the SAR parameter change causes a variation of theinterference pattern parameter, such as the Fresnel zone plate focallength or the Fourier slice scaling parameter for example. In this case,the SAR raw data correcting module 44 is adapted to determine a scalingfactor in accordance with the parameter change and to apply the scalingfactor to the SAR raw data in order to obtain the corrected SAR data.The scaling of the SAR raw data can be performed via memory addressingoperations.

In another embodiment, the SAR parameter change causes a Doppler shift.In this case, the SAR raw data correcting module 44 is adapted todetermine a phase factor and to apply the phase factor to the SAR rawdata in accordance with the method described above.

Alternatively, the SAR raw data correcting module 44 can be adapted togenerate a SAR digital pattern such as the one graphically representedin FIG. 3A using the received SAR raw data. The SAR raw data correctingmodule 44 is further adapted to determine an azimuth shift using the SARparameter change or the Doppler shift in accordance with the methodsdescribed above. The SAR raw data correcting module 44 is also adaptedto shift the columns of the SAR digital pattern in accordance with thedetermined azimuth shift in order to obtain a corrected SAR patterncorresponding to the SAR corrected data. In this case, the corrected SARpattern is sent to an optical modulator comprising a light modulator andthe transmittance of the light modulator is controlled in accordancewith the corrected SAR pattern representing the SAR corrected data.

In one embodiment, the SAR raw data correcting module 44 is adapted tocompensate for changes of SAR parameters which cause a variation of theinterference pattern parameter and for changes of SAR parameters whichcause a Doppler shift. In this case, the SAR raw data correcting module44 is adapted to determine the adequate correcting method using the SARparameter type.

In one embodiment, the memory 42 comprises a database of empirical datawhich contains values for the scaling factor, the phase factor, and/orthe azimuth shift as a function of values of the SAR parameter change,the interference pattern parameter such as the Fresnel zone plate focallength or the Fourier slice scaling parameter for example, and/or theDoppler shift.

In one embodiment, the SAR raw data processing system 40 is adapted toreceive a value for the SAR parameter and the SAR raw data correctingmodule 44 is adapted to determine the value of the SAR parameter changeusing the received value of the SAR parameter and a reference SARparameter value.

In one embodiment, the SAR image generator 46 comprises a processingunit adapted to digitally generate a SAR image of the target area usingthe corrected SAR data. In this case, the processing unit is configuredfor applying the adequate methods for digitally creating a SAR imagedescribed above. It should be understood that, when the SAR imagegenerator 46 is a digital image generator, the SAR data correctingmodule 44 and the SAR image generator 46 can be embodied in a singlemodule 48 comprising at least one processing unit configured forcorrecting the SAR raw data and generate a SAR image of the correctedSAR data.

In another embodiment, the SAR image generator 46 comprises a coherentlight source, a pixel drive unit, a light modulator such as an SLM or amicro display, and an optical processor. Any adequate light modulatorcan be used. For example, the light modulator can be a liquid crystaldisplay, a micro mirror SLM, an electro-optic SLM, a magneto-optic SLM,or the like. The pixel drive unit is adapted to control thetransmittance of each addressable pixel of the light modulator. Thepixel drive unit receives the corrected data in the form of a complexfunction or a SAR pattern from the SAR raw data correcting module 44 orthe memory 42, and sets the transmittance of the addressable pixels ofthe light modulator in accordance with the SAR corrected data. Thecoherent light source emits a coherent light beam which illuminates thelight modulator. The coherent light incident to the light modulator ismodulated in accordance with the transmittance of the pixels and amodulated light corresponding to an image of the SAR corrected data istransmitted at the output of the light modulator. The modulated lightthen propagates through the optical processor which generates areconstructed and corrected image of the target area. The image can bedisplayed on a screen. The SAR image generator can also be provided withan optical detector or sensor, such as a CCD or a CMOS sensor, forconverting the optical reconstructed image of the target area into adigital image which can be saved in memory 42.

In one embodiment, the coherent light source comprises a spatial filterto improve the quality of the image. A polarizer may be provided betweenthe light modulator and the coherent light source if the light modulatorrequires polarized light.

In one embodiment, the optical processor comprises at least onecylindrical lens to selectively focalize the azimuth or range field. Thecylindrical lens can be used together with a spherical lens to providefocusing power in the azimuth or range direction. The cylindrical lensmay also compensate for a chirp along the range direction and/or for arange phase factor along the azimuth direction.

In one embodiment in which the SAR image generator comprises a lightdetector, the optical processor can comprise at least one spherical lensfor imaging the processed wave to the detector plane of the lightdetector.

While the present description refers to the compensation of a variationof Fresnel zone plate focal length for a stripmap SAR and thecompensation of a variation of Fourier slice scale parameter for aspotlight SAR, it should be understood that the method and apparatusdescribed above can be used for compensating for a variation of anyinterference pattern parameter for any type of SAR.

It should be understood that the method 20 and the system 40 can be usedwith interferometric SARs and non-interferometric SARs.

While the present description refers to a synthetic aperture radar, itshould be understood that the methods, apparatuses, and systemsdescribed above can be applied to any adequate synthetic apertureimaging system. For example, the method 20 and the apparatus 40 can beused with a synthetic aperture SONAR (SAS), a synthetic aperture LIDAR,a synthetic aperture terahertz system, a synthetic aperture infraredsystem, or the like.

It should be noted that the present invention can be carried out as amethod or can be embodied in a system or an apparatus. The embodimentsof the invention described above are intended to be exemplary only. Thescope of the invention is therefore intended to be limited solely by thescope of the appended claims.

The invention claimed is:
 1. A method for processing data generated by a synthetic aperture imaging system, comprising the steps of: receiving raw data representative of return signal echoes from a target area to be imaged; receiving a parameter change causing a Doppler shift for said synthetic aperture imaging system, said parameter change having been performed before acquisition of the raw data, said parameter change comprising a change of one of a squint angle and an eccentricity of a platform orbit; using a processing unit, digitally correcting said raw data in accordance with said parameter change, thereby compensating for said parameter change in order to obtain corrected data; and generating an image of said target area using said corrected data, said generating comprising: generating an incident light; modulating said incident light in accordance with said corrected data, thereby obtaining a modulated light; and optically processing said modulated light, thereby obtaining an optical image of said target area.
 2. The method as claimed in claim 1, wherein said digitally correcting said raw data comprises: determining a phase factor corresponding to said parameter change; and applying said phase factor to said raw data, thereby obtaining said corrected data.
 3. The method as claimed in claim 1, further comprising the step of detecting said optical image to obtain a digital image of said target area.
 4. The method as claimed in claim 1, wherein said digitally correcting said raw data comprises: generating a SAR pattern using said raw data, said SAR pattern comprising data points organized along an azimuth axis and a range axis, each one of said data points being associated with an amplitude value and a phase value; determining an azimuth shift in accordance with said parameter change; and translating said data points of said SAR pattern in accordance with said azimuth shift, thereby obtaining a corrected SAR pattern representing said corrected data.
 5. The method as claimed in claim 1, wherein said receiving said raw data comprises receiving said raw data from one of a synthetic aperture radar system, a synthetic aperture sonar system, a synthetic aperture lidar system, a synthetic aperture terahertz system, and a synthetic aperture infrared system.
 6. A system for generating a synthetic aperture image of a target area, comprising: a memory configured to store raw data representative of return signal echoes from said target area and a parameter change causing a Doppler shift for said synthetic aperture imaging system, said parameter change having been performed before acquisition of the raw data, said parameter change comprising a change of one of a squint angle and an eccentricity of a platform orbit; a processing unit configured to correct said raw data in accordance with said parameter change in order to compensate for said parameter change and obtain corrected data; and an image generator configured to generate an image of said target area using said corrected data, said image generator comprising: a light source configured to generate incident light; a light modulator configured to modulate said incident light in accordance with said corrected data, thereby obtaining a modulated light; and an optical processor configured to optically process said modulated light, thereby obtaining said image of said target area.
 7. The system as claimed in claim 6, wherein said processing unit is configured to: determine a phase factor corresponding to said parameter change; and apply said phase factor to said raw data in order to obtain said corrected data.
 8. The system as claimed in claim 6, wherein said image generator further comprises an image sensor configured to detect said optical image and to generate a digital image of said target area.
 9. The system as claimed in claim 6, wherein said processing unit is configured to: generate a SAR pattern using said raw data, said SAR pattern comprising data points organized in accordance with an azimuth axis and a range axis, each one of said data points being associated with an amplitude value and a phase value; determine an azimuth shift in accordance with said parameter change; and translate said data points of said SAR pattern in accordance with said azimuth shift in order to obtain a corrected SAR pattern representing said corrected data.
 10. The system as claimed in claim 6, wherein said memory is configured to store said raw data generated by one of a synthetic aperture radar system, a synthetic aperture sonar system, a synthetic aperture lidar system, a synthetic aperture terahertz system, and a synthetic aperture infrared system. 